Noise cancelling microphones are used to reduce ambient background noise in headsets with microphone booms.
The performance of a noise-cancelling microphone depends on its positioning relative to the headset user's mouth—it is calibrated to one particular distance and angle relative to the mouth. When it is incorrectly positioned, e.g., when the microphone boom is directed below or above the mouth, the speech pickup characteristics, such as the mouth-to-line transfer function, change. The sensitivity is significantly lowered, meaning that transmitted speech is unacceptably soft. Noise pickup on the other hand is relatively unaffected by mispositioning of the microphone, leading to a decreased signal-to-noise ratio in the transmitted signal. The frequency response of the speech pickup may also change due to the mispositioning, the lower frequencies of the transmitted speech being attenuated relative to the higher frequencies.
The fundamental limitation of a noise-cancelling microphone lies in the fact that the spatial sensitivity is fixed at production. If due to mispositioning of the microphone boom, user speech does not originate from the predetermined position, i.e. distance and direction relative to the microphone assembly, the signal-to-noise ratio of the transmitted signal will be suboptimal. In the following positioning refers to distance between the mouth and the microphone assembly as well as the orientation of the microphone assembly.
An omnidirectional microphone is less sensitive to positioning. This means that in cases of incorrect microphone boom positioning, it is disadvantageous to use a noise-cancelling microphone relative to using an omnidirectional microphone.
Experience shows that users of headsets often position their microphone boom incorrectly, hence the need for an alternative solution.
Dual microphone DSP solutions, termed beamformers in the following, consisting of two omnidirectional microphones in a microphone assembly may replace and improve on a noise cancelling microphone. This is done in great part by maintaining an adaptive spatial sensitivity to fit all or some positionings of the microphone boom/microphone pair. Typical omni-directional microphones used in such systems are produced with a variance of the amplitude and phase response of the individual microphones. In addition, the microphone responses change unpredictably across time in response to temperature, humidity, mechanical shocks and other factors (drift). The response variance cannot be ignored if satisfactory noise cancelling performance is to be achieved. Depending on the specific noise cancelling application, the variance of microphone sensitivities may be handled in one of two ways, representing different problem sets:                1. Microphone sensitivities are calibrated by some process which requires one or more active sound sources at a known position with respect to distance and/or angle. Calibration may occur at production or when the system is in use. A calibration fixture may be used as part of the manufacturing process. User speech may be used if the microphone boom/noise cancelling microphone is in a known position relative to the mouth. Background noise may be used if certain characteristics about it are known. This approach does not handle drift.        2. Use a system which inherently works optimally for all instances of microphone sensitivies and positions of microphone boom/microphone pair but does not explicitly or implicitly compute the position or mispositioning. It does not rely on a sound source at known position at any time for calibration purposes, because such a situation does not occur in lifetime of the noise cancelling application. Since microphone sensitivity and position of the microphone boom/microphone pair are convolved and inseparable effects (see later), it is impossible to explicitly or implicitly extract knowledge from the observed signals of the microphone sensitivities or the position of the microphone boom/microphone pair.        
U.S. Pat. No. 7,346,176 (Plantronics) and U.S. Pat. No. 7,561,700 (Plantronics) disclose a system and method which detects whether or not a microphone apparatus is positioned incorrectly relative to an acoustic source and of automatically compensating for such mispositioning. A position estimation circuit determines whether the microphone apparatus is mispositioned. A controller facilitates the automatic compensation of the mispositioning. This system and method requires pre-calibration of the microphones.
U.S. Pat. No. 8,693,703 (GN Netcom) discloses a method of combining at least two audio signals for generating an enhanced system output signal is described. The method comprises the steps of: a) measuring a sound signal at a first spatial position using a first transducer, such as a first microphone, in order to generate a first audio signal comprising a first target signal portion and a first noise signal portion, b) measuring the sound signal at a second spatial position using a second transducer, such as a second microphone, in order to generate a second audio signal comprising a second target signal portion and a second noise signal portion, c) processing the first audio signal in order to phase match and amplitude match the first target signal with the second target signal within a predetermined frequency range and generating a first processed output, d) calculating the difference between the second audio signal and the first processed output in order to generate a subtraction output, e) calculating the sum of the second audio signal and the first processed output in order to generate a summation output, f) processing the subtraction output in order to minimise a contribution from the noise signal portions to the system output signal and generating a second processed output, and g) calculating the difference between the summation output and the second processed output in order to generate the system output signal.
Thus, it remains a problem to obtain robust and optimal noise cancellation in a headset regardless of the position of the microphones using uncalibrated microphones.